Isolated lissencephaly (ILS) is a human brain developmental disorder in which the brain has a smooth cerebral surface and disorganized cortical layers that is thought to be due to abnormal neuronal migration. ILS is often associated with haploinsufficiency at chromosome 17p13, or with heterozygous mutations in LIS1 or PAFAH1B1, a subunit of platelet activating factor acetylhydrolase. Miller-Dieker syndrome (MDS) is associated with larger deletions and consists of lissencephaly and more complex phenotypes including facial abnormalities. We have disrupted three genes (Lis1, Mnt and 14-3-3_) in the syntenic region of the mouse (chromosome 11B2) to understand the functions of these genes, and to determine if they contribute to MDS. In addition, we are using these mice to create deletions spanning the MDS critical region in mouse using the Cre-loxP system. To further understand the function of Lis1, we produced three different Lis1 mutant alleles in the mouse. Homozygous null mice died early in embryogenesis soon after implantation. Mice with one inactive allele displayed cortical, hippocampal, and olfactory bulb disorganization resulting from delayed neuronal migration by a cell autonomous neuronal pathway. Mice with further reduction of Lis1 gene activity displayed more severe brain disorganization as well as cerebellar defects. Our results demonstrate an essential, dosage sensitive neuronal- specific role for Lis1 in neuronal migration throughout the brain and, when completely absent, in early embryonic development. These phenotypes are distinct from those of other mouse mutants with neuronal migration defects, suggesting that Lis1 participates in a novel pathway for neuronal migration. When Lis1, Mnt and 14-3-3_ disruptions were made, loxP sites were introduced into these genes to allow deletions to be made with Cre. We are using these single gene mutants to create small (170 kb) and large (800 kb-1 Mb) deletions to determine the contribution of genomic regions and individual genes to the MDS phenotype.